Every leader has positive and negative qualities. This is because few leaders are absolutely terrible and no leaders are absolutely perfect. When assessing someone's leadership skills, then, you need to look at both positive and negative qualities, and weigh them to see if your leader is ultimately an effective or ineffective.
Leaders tell people what to do and how to act. This means that leaders need to have good character; they need to act with integrity and stand behind their words. This is because actions ultimately speak louder than words. If a leader is constantly telling his subordinates to watch their budgets and ensure that they don't go over their limits, while also flaunting the fact that he uses his expense account for non-business-related things, then his actions are speaking louder than his words. So, a positive quality is standing behind your rules and regulations; ignoring them is a negative quality.
Leaders need to understand their subordinates. This means listening to them when they talk; it means taking their ideas and complaints on board and responding to them. It doesn't necessarily mean that a good leader has to do everything her subordinates ask of her, but it does mean that she needs to at least explain why an idea is or is not viable. So, good listening and responding is a positive aspect of leadership; poor listening and responding is a negative aspect.
The key element of a leadership job is in making decisions. These vary between day-to-day decisions and long-term, wide-reaching decisions such as which health plan an office is going to go with. So, the ability to look at information and quickly make a clear, rational decision is a positive quality of leadership. A tendency to hem and haw, mull things over or put difficult decisions out of sight (but never out of employees' minds) is a negative quality of leadership.
While a leader does run a group of people, he is also part of it. This means that good leaders act as if they are part of the team rather than acting as if the team is simply an auxiliary part of their job. While leaders are the key decision-makers, they should still involve people in things that affect them. They should also approach things as a team: taking credit for subordinate work, for example, is not very good teamwork. Positive leaders recognize the team they are part of, while negative leaders use the team as tools for their own ends.